1. Field of the Invention
The present invention relates to a new and improved process for synthesizing zeolite beta. More particularly it relates to a hydrothermal synthesis of zeolite beta in which the organic directing agent is the tetraethylammonium ion imparted to the reaction mixture in the form of a tetraethylammonium halide, preferably the bromide salt. The essential high degree of alkalinity of the reaction mixture is provided by a combination of NaOH and diethanolamine. Optionally seed crystals of zeolite beta are employed to shorten the crystallization period.
2. Discussion of the Prior Art
Zeolite beta and the method for its preparation were first described in U.S. Pat. No. 3,308,069, issued Mar. 7, 1967, to R. L. Wadlinger et al. As reported therein, zeolite beta has a chemical composition in its as-synthesized form expressed in terms of molar oxide ratios of EQU [xNa.sub.2 O+(1.0.+-.0.1 -x) TEA.sub.2 O]:Al.sub.2 O.sub.3 :ySiO.sub.2 :aH.sub.2 O
wherein "x" has a value of less than 1.0, "y" has a value of greater than 10 but less than 200 and "a" has a value of up to 4 depending upon the degree of hydration of the zeolite. In the formula, "TEA" represents the tetraethylammonium cation. The zeolite is said to be formed by crystallization at elevated temperatures in the range of 75.degree. C. to 200.degree. C. from an aqueous reaction mixture containing tetraethylammonium hydroxide (TEAOH) as the alkali and reactive sources of Na.sub.2 O, Al.sub.2 O.sub.3 and SiO.sub.2. The composition of the reaction mixture, expressed in terms of mol ratios of oxides, falls within the following ranges
SiO.sub.2 /Al.sub.2 O.sub.3 --from about 10 to about 200 PA1 Na.sub.2 O/TEA.sub.2 O--from about zero to 0.2 PA1 TEA.sub.2 O/SiO.sub.2 --from about 0.05 to about 0.5 PA1 H.sub.2 O/TEA.sub.2 O--from about 40 to about 150 PA1 SiO.sub.2 /Al.sub.2 O.sub.3 --5 to 250 PA1 H.sub.2 O/SiO.sub.2 --10 to 100 PA1 OH.sup.- /SiO.sub.2 --0.09 to 0.8 PA1 R/(R+M)--0.1 to 0.8 PA1 SiO.sub.2 /Al.sub.2 O.sub.3 --20 to 50 PA1 H.sub.2 O/SiO.sub.2 --10 to 200 PA1 OH.sup.- /SiO.sub.2 --0.10 to 2.0 PA1 M.sup.+ /SiO.sub.2 --0.05 to 1.0 PA1 R/SiO.sub.2 --0.10 to 2.0
The more significant, i.e., characterizing, interplanar d-spacings of zeolite beta, dried in air at 110.degree. C., are shown in Table A, below:
TABLE A ______________________________________ Interplanar Relative d-Spacing Intensity (A) (I/I.sub.o) ______________________________________ 11.5 .+-. 0.4 M-S 7.4 .+-. 0.2 W 6.6 .+-. 0.15 W 4.15 .+-. 0.10 W 3.97 .+-. 0.10 VS 3.00 .+-. 0.07 W 2.05 .+-. 0.05 W ______________________________________
In a later reported synthesis of zeolite beta reported in U.S. Pat. No. 4,554,145, M. K. Rubin, the organic directing agent employed was derived from dibenzyl - 1,4 -diazabicyclo [2,2,2] octane chloride. The synthesis involved the preparation of a reaction mixture containing sources of alkali metal oxide, an oxide of aluminum, an oxide of silicon, water and the cation of the aforesaid organic compound in proportions within the following ranges:
wherein R represents the organic cation and M represents the alkali metal cation. The quantity of OH.sup.- ion is calculated using only the contribution of inorganic alkali. Crystallization of the zeolite beta was accomplished in the two specific preparations disclosed at 99.degree. C. and 100.degree. C. over periods of 119 and 169 days, respectively. Zeolite P and mordenite types of crystalline impurities were found in the products along with zeolite beta.
In U.S. Pat. No. 4,642,226, issued Feb. 10, 1987, to R. B. Calvert et al, the synthesis of zeolite beta using the dibenzyldimethylammonium ion as the directing agent is reported. The organic cations are derived from dibenzyldimethylammonium chloride added to the reaction mixture per se or produced therein in situ by the reaction of benzyl chloride with dimethylbenzylamine. It is stated that the composition of the reaction mixture is critical with respect to the presence of alkali-derived OH.sup.- groups and the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio. The latter cannot be greater than 250 and the former, as determined by the ratio OH.sup.- /SiO.sub.2, cannot be less than 0.1 to prevent the formation of another unspecified silicate as the principal crystalline product. The broad composition ranges for the reaction mixture are disclosed as:
wherein R represents the dibenzyldimethylammonium ion and M.sup.+ represents an alkali metal ion from which the OH.sup.- ion concentration is calculated exclusively. Maintaining the reaction mixtures at 80.degree. C. to 175.degree. C. for 24 hours to 120 days is said to be required to produce crystals of zeolite beta. At 100.degree. C. a digestion, i.e., crystallization, period of 115 days was found to produce a product containing 55% zeolite beta.
What is believed to be the most recently reported synthesis of zeolite beta is reported in U.S. Pat. No. 4,923,690, Valyocsik et al. In accordance with the process disclosed, a mixture of tetraethylammonium halide and tetraethylammonium hydroxide is disclosed as a feasible crystallization directing agent. Either the hydroxide or the halide salt used alone is also reported to be suitably employed to yield a zeolite beta-containing product which is 30% to 90% crystalline of which, presumably, zeolite beta can constitute essentially all or some minor portion thereof.